Virtually every aspect of an organism's phenotype undergoes modification in its functionality and morphology during aging. The complexity of organisms and the vast amount of aging related phenomena make it very difficult to distinguish between phenotypes that cause aging and which are the effect of it. In colonial organisms, like Botryllus schlosseri, individuals originally derived, like us, by sexual reproduction and chordate development can metamorphose to clonal founders that undergo weekly formation of new individuals by budding from a small group of stem cells. Individuals are transient structures which die through massive apoptosis and successive buds mature to replicate an entire new body, every week. As a result, their stem cells, which are the only self renewing cells in a tissue, are the only cells which remain through the entire life of the genotype and are the only cells that can retain the effects of time. Therefore, aging of the colony in this organism is, by definition, aging of the stem cells: every other cell is regenerated from them on a weekly basis. In this colonial model organism, we can clearly define that stem cell aging is the root cause of senescence of the entire colony. Most importantly, we can sample single tissue or gonad stem cells over the dozens of asexual doublings the colony undergoes while aging, and follow the serial changes or clonal selections that occur in the stem cell pools for each tissue, and for the germline. In this proposal, we outline studies to investigate the cellular and molecular mechanisms of aging and regeneration in tissues of B.schlosseri. Specifically, we propose to characterize the molecular pathways which underline sexual versus asexual reproduction in a colonial chordate; to identify and characterize the molecular mechanisms associated with robust regeneration activity and tissue homeostasis and to test their effects on longevity, and age-related cellular and molecular processes; and to investigate the effect of exposure of old colonies to circulating factors from young colonies (through vascular anastomosis), on their molecular expression profile and regeneration potential. We will use the Illumina high throughput, mRNA sequencing platform to compare differential expression of genes between: asexual developmental pathways in young colonies versus old. This analysis includes comparing long and short lived genotypes and within groups-genotypes that demonstrate simultaneous versus random death. We will also compare expression profiles of enriched stem cells and stem cell niches from defined tissues and organs over time. This approach with inbred, self crossed lines or samples of the same genotype will allow identification of genes which are expressed differentially over the life span of the tested genotypes that are likely to affect aging and alter tissue regeneration capacities. We will identify the specific cells that show the differential expression by in situ hybridization. This comprehensive screen will be followed by functional experiments that evaluate gene function by knockdown studies, and altered biology in situ as they migrate across vascular bridges between colonies to participate in budding and germ cell formation. We will also determine, at the single cell level the aging signatures in both stem cells and niche cells.